Vehicles require transmissions in order to convert torque and rotational speed. The task of a transmission is to convert the traction force produced by a drive assembly. The present invention relates to a method for operating an automatic transmission.
In the spirit of the present invention, the term “automatic transmission” shall apply to all synchronized transmissions and all unsynchronized transmissions with automated gear changing or automatic gear changing. In an automatic transmission with automated gear changing, gears are normally changed with an interruption in torque flow. However, in an automatic transmission with automatic gear changing, gears are normally changed without an interruption in torque flow. Automated constant-mesh countershaft transmissions or automated synchronized transmissions, for example, are included in the category of transmission with automated gear changing. Conventional, fully automatic transmissions belong to the category of transmission with automatic gear changing.
Furthermore, the term “automatic transmission” shall also apply to transmissions that have a plurality of transmission elements. Transmissions of this kind with a plurality of transmission parts are also called range-change transmissions, wherein either each transmission part of a range-change transmission is synchronized, or each transmission part of a range-change transmission is unsynchronized, or also at least one transmission part of a range-change transmission is synchronized, and the or all other transmission parts of the range-change transmission are designed to be non-synchronized.
In automatic transmissions known from practical application, subject to the actuation of a gas pedal of the motor vehicle, a shifting strategy is determined and a gear for the automatic transmission is selected accordingly. Then, when the motor vehicle coasts, or as the case may be coasts to a stop, with a non-actuated gas pedal, coast downshifts are carried out by a first group of automatic transmissions known from practical application. Such coast downshifts result in continual gear changes during coasting, or as the case may be coasting to a stop, however these are perceptible to the driver of the motor vehicle as changes in deceleration, because the internal combustion engine, after every coast downshift, runs through a relatively wide rotational speed range, and the trailing throttle torque thereof depends on the rotational speed. Such strongly varying decelerations of the motor vehicle resulting from such coast downshifts, along with the related noise development, are perceived as uncomfortable.
In order to avoid the disadvantages described above, automatic transmissions are also known from practical application that forego coast downshifts during coasting, or as the case may be coasting to a stop. In this second group of automatic transmissions, the automatic transmission, during coasting and also with an engaged starting clutch, remains in the gear in which the motor vehicle had previously been driven with an actuated gas pedal. Here however, there is continual monitoring of whether the transmission input speed falls below a limit value that approximates the idling speed of the internal combustion engine, and if the speed falls below this limit value, the starting clutch is disengaged in order to avoid trailing throttle or pushing operation of the internal combustion engine. The automatic transmission remains in the original gear, with disengaged starting clutch, however this can lead to an undesirable situation in which, when the starting clutch is subsequently engaged, for example as a result of actuating the gas pedal, a gear that does not match the vehicle's speed is selected by the automatic transmission. Then downshifts must first be carried out by the automatic transmission in order to meet the request for acceleration expressed by the actuation of the gas pedal.
However, these kinds of downshifts take a great deal of time, so that a reaction to the actuation of the gas pedal can only take place with some delay. This lack of spontaneity is also perceived as a disadvantage.
DE 10 2004 038 520 A1 discloses a method for operating an automatic transmission, wherein according to this prior art, when coasting or as the case may be coasting a motor vehicle to a stop, downshifts can be carried out while avoiding load changes during a downshift.